DE112004002324B4 - Method of reducing NOx in diesel engine exhaust - Google Patents

Abstract

A method of reducing nitrogen oxides, including NO, in an exhaust gas stream also containing oxygen, carbon monoxide and hydrocarbons at a temperature above 150 ° C, the method comprising:
Passing air through a non-thermal plasma reactor to produce an ozone-containing plasma and supplying the plasma to the exhaust gas stream for the oxidation of NO to NO ₂ ,
Supplying ethanol to the exhaust stream separately from the supply of the plasma for the reduction of nitrogen oxides and
then contacting the exhaust gas stream with a two-pot reduction catalyst containing in the first bed NaY zeolite and / or BaY zeolite and in the second bed CuY zeolite to reduce the nitrogen oxides to N ₂ .

Description

TECHNICAL AREA

The present invention relates generally to the treatment of exhaust gas from a hydrocarbon powered fuel source, such as a diesel engine, which is operated with a fuel lean combustion mixture. In particular, this invention relates to treating the NO _x content of exhaust gas by the separate additions of ethanol and plasma-treated air before the exhaust gas comes into contact with a base metal-exchanged zeolite catalyst.

BACKGROUND OF THE INVENTION

Diesel engines, some gasoline fueled engines, and many hydrocarbon fueled power plants are operated at fuel mass ratios at higher than stoichiometric air for improved fuel economy. However, such lean-burn engines and other power sources produce a hot exhaust gas with a relatively high content of oxygen and nitrogen oxides (NO _x ). In the case of diesel engines, the temperature of the exhaust gas from a warmed-up engine is typically in the range of 200 ° C to 400 ° C and typically has a volume composition of about 17% oxygen, 3% carbon dioxide, 0.1% carbon monoxide , 180 ppm hydrocarbons, 235 ppm NO _x with the remainder nitrogen and water.

These NO _x gases, typically containing nitrogen monoxide (NO) and nitrogen dioxide (NO ₂ ), are difficult to reduce to nitrogen (N ₂ ) due to the high oxygen (O ₂ ) content in the hot exhaust gas stream. It is therefore an object of the present invention to provide an improved method for reducing NO _x in such gas mixtures.

In the US 6,546,717 B1 discloses a method for purifying exhaust gases from a diesel engine, wherein the exhaust gas flow through a corona discharge tube or a high energy lamp is supplied to ozone, the exhaust gas is also fed ammonia or propene and the pretreated exhaust gas stream is then contacted with a Einbeteduktionskatalysator.

From the DE 100 21 693 A1 A method of purifying exhaust gases from diesel engines is known in which the exhaust gas is first passed through a hydrocarbon adsorber before the thus pretreated exhaust gas is exposed to a non-thermal gas discharge plasma, then admixed with ammonia and finally contacted with a single pass reduction catalyst.

In the EP 0 526 896 A1 For example, there is described a catalyst for purifying exhaust gas comprising a first zeolite catalyst ion-exchanged with cobalt and at least one alkaline earth metal and a second zeolite catalyst ion-exchanged with copper.

In the DE 196 80 576 T1 discloses a method for treating an exhaust gas stream to remove nitrogen oxides, wherein the first added to the exhaust gas with air added ethanol before the thus pretreated exhaust gas flow is passed through a turbocharger and then contacted with a Einbettreduktionskatalysator.

SUMMARY OF THE INVENTION

The present invention provides a method of reducing NO _x in an exhaust gas stream using certain base metal exchanged zeolite Y catalysts with separate upstream additions of ozone and ethanol. This method is suitable for reducing NO _x in a hot exhaust gas stream which also contains a large amount of oxygen. The present invention is suitable for treating exhaust gas streams generated in lean-burn power plants and engines, such as diesel engines. The present invention will be illustrated with reference to the application of diesel engines.

In the present invention, the exhaust NO _x containing ultimately is passed through or over a two bed catalyst in which the upstream bed of sodium Y zeolite or barium Y zeolite and the downstream bed is copper Y zeolite. These base metal exchanged zeolite catalysts are sometimes referred to in this specification as NaY, BaY, or CuY, respectively. The effectiveness of the twin catalyst is firstly due to the upstream addition of air plasma generated ozone to the Exhaust gas to convert NO to NO ₂ , and secondly, by the separate addition of ethanol to the exhaust to promote the reduction of NO ₂ to N ₂ , enhanced. A plasma reactor is used at lower catalyst temperatures so that the ozone converts the NO to NO ₂ while a portion of the ethanol is oxidized to acetaldehyde. The energy density requirement of the plasma reactor is a function of the temperature of the twin catalyst reactor (or the closely related exhaust gas temperatures of the reactor inlet). The ethanol is added in an amount sufficient to reduce the NO ₂ content of the exhaust gas in a temperature based on the inlet of the twin catalyst or in the reactor. As the exhaust gas then flows through the twin catalyst, a portion of the ethanol is oxidized to acetaldehyde and both participate in the reduction of NO ₂ to N ₂ . It has been found that the ethanol and acetaldehyde are effective for reducing NO ₂ over the catalyst surface without undue degradation of catalyst activity.

Ozone is suitably generated by passing ambient air through a high efficiency non-thermal plasma generator. In a preferred embodiment, the plasma generator is a tube having a dielectric cylindrical wall forming a reactor space. A linear high voltage electrode is disposed in this reactor space along the axis of the tube. An outer ground electrode comprising an electrically conductive wire is spirally wound around the cylindrical dielectric wall. The application of a high-frequency alternating voltage to the central electrode generates a plasma in the ambient air flowing through the reactor. The combination of the spiral grounding electrode and the linear axial electrode creates intertwined helical regions of active and passive electric fields. Significantly, ozone and possibly other chemically activated oxygen ions and / or radicals and other species are formed in the air stream flowing through the plasma reactor. As stated previously, the ejection of the plasma reactor enters the exhaust stream and the ozone oxidizes NO to NO ₂ . Downstream, the NO ₂ is reduced to N ₂ through the reaction with ethanol and acetaldehyde over the base metal divalent catalyst.

The temperature of the exhaust gas entering the two beds of the reduction catalyst (or a temperature of the bed itself) is monitored and the NO _x content of the exhaust gas is measured or estimated. Both the plasma power density and the molar ratio EtOH / NO are described in more detail in the following description _x is set based on such reduction catalyst temperature and / or controlled. In general, the demand for plasma energy decreases with increasing reduction catalyst temperature, whereas the requirement for ethanol increases with increasing catalyst temperature. In one operated in steady state power plant the catalytic reactor can operate for long periods at a constant temperature and the NO _x content may remain substantially the same. In this case, there is no need for frequent monitoring of the set plasma energy density and the EtOH / NO _x ratio. However, the conditions during warm-up and the conditions during different driving in diesel or lean-burn gasoline engine engines are likely to require more frequent monitoring or estimation of the NO _x content of the exhaust gas and the catalyst temperature.

For a reducing catalyst temperature in a range between 200 ° C and 400 ° C, the process according to the present invention is capable of achieving an average NO _x to N _{2 conversion} of 90% through extended operation of the base metal divalent catalyst.

The exhaust leaving the diesel engine contains unburned hydrocarbons, especially diesel particulates, as well as carbon monoxide, which are preferably removed by catalytic oxidation and filtration of the exhaust gas prior to ozone addition to the exhaust gas.

Other objects and advantages of the present invention will become apparent from the following description of a preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

1 FIG. 12 is a schematic, one preferred method of NO _x reduction, in accordance with the present invention, of a diesel engine exhaust gas flow diagram. FIG.

2 FIG. 12 is a side elevational view in partial cross section of a non-thermal plasma reactor used to treat ambient air to produce ozone according to the practice of the present invention. FIG.

3 FIG. 12 is a plot of NO _x conversion efficiency over a base metal exchanged Y zeolite two-reduction catalyst reactor as a function of time of catalyst exposure to exhaust gas flow using propylene (dash-dotted line), gasoline (solid line), or ethanol (dotted line). FIG reducing agent.

4 Figure 12 shows a plot of plasma energy density (joules per liter) of plasma versus reduction catalyst temperature and a graph of the feed ratio of ethanol versus NO _x as a function of the reduction catalyst temperature.

5 Figure 12 is a graph of NO _x conversion (%) versus reduction catalyst temperature over a range of 150 ° C to 400 ° C for a twin-bed reactor.

6 Figure 4 is a schematic flow diagram of an experimental sidestream plasma catalyst system in engine dynamometer tests.

7 is a diagram showing the advantages of side stream addition of plasma treated air. The solid line shows NO _x conversion with plasma addition and the dotted line shows NO _x conversion without plasma addition.

8th FIG. 13 is a bar graph of NO _x conversion at reducing catalyst inlet temperatures between 200 ° C and 300 ° C. FIG. The bar data compare NO _x conversions in the two cases where (a) ethanol is injected into the exhaust stream immediately upstream of the plasma addition and (b) ethanol is injected into the exhaust stream downstream of the plasma addition.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An application of the present invention is schematically shown in FIG 1 illustrated. The box 10 represents a diesel engine and the line 12 embodies the flow of exhaust gas from the diesel engine 10 , Diesel engines are typically operated at air to fuel mass ratios that are significantly higher than the stoichiometric ratio of air to fuel, and the exhaust contains a significant amount of unreacted O ₂ and N ₂ (from the air). The temperature of the exhaust gas from a warmed engine is typically in a range between about 200 ° C and about 400 ° C. The application of the present invention will be described for the case of a diesel engine, but it should be understood that the method could be used to remove the exhaust gas from a lean burn gasoline engine or the exhaust gas of any hydrocarbon fueled power plant which has an excess of air used to burn the fuel, treat it. In diesel engine exhaust, in addition to O ₂ and N ₂ , the hot gas contains CO, CO ₂ , H ₂ O, and hydrocarbons (some in particulate form) that are not completely combusted. The constituent of the exhaust gas to which the object of the present invention is applicable, however, is the mixture of nitrogen oxides (mostly NO and NO ₂ with a trace N ₂ O, collectively referred to as NO _x ), which is formed by reaction of N ₂ and O ₂ in the combustion cylinders of the engine (or power plant) are formed. The content of NO _x in diesel exhaust is typically about 200 to 300 parts per million (ppm). Accordingly, the purpose of the present invention is to treat nitrogen oxides, which account for approximately 2-3 / 10,000ths of the volume of the exhaust gas.

Again with reference to the 1 produces a diesel engine 10 an exhaust stream which is first passed through a two-pass oxidation catalyst reactor 14 passed and ultimately through a reduction catalyst reactor 24 is directed. The exhaust stream is treated along its flow path and its composition changed. The Indian 1 when 12 designated, leaving the engine exhaust stream flows through the oxidation catalyst reactor 14 in which palladium (Pd) is a suitable catalyst. The function of the oxidation catalyst reactor 14 is to complete the oxidation of CO to CO ₂ as well as the oxidation of hydrocarbons including particles to CO ₂ and to H ₂ O. Some of the hydrocarbons can be converted to aldehydes by a partial oxidation process. Acetaldehyde (AA) is a typical aldehyde product. The oxidation reactor 14 does not reduce the amount of NO _x in the exhaust gas. The now with 16 oxidized exhaust gas stream becomes the reduction catalyst reactor 24 directed. As will be described below, that in the reactor 24 For example, the reduction catalyst employed is designed specifically for the reduction of NO _x under slightly lean, excess oxygen-containing conditions, and the reaction is sometimes referred to as lean NO _x selective catalyst reduction (SCR). In accordance with this invention, the diesel engine exhaust is treated with two separate side streams before it enters the reduction catalyst reactor 24 entry.

A stream of compressed ambient air, suitably withdrawn from the engine compartment, passes through an efficient tubular non-thermal plasma reactor 20 blown to convert a portion of the oxygen in the air to ozone and possibly other activated oxygen species. The ozone-containing plasma is added to the exhaust gas to oxidize NO to NO ₂ . After the plasma addition, the diesel exhaust gas from a reservoir 26 Ethanol is added separately in a manner which will be explained later in this specification, wherein the ethanol serves as a reducing agent for NO ₂ . Residual ozone may oxidize some ethanol to acetaldehyde, which also serves as a reducing agent for NO and NO ₂ . In the 1 the exhaust gas stream after the addition of the ozone-containing plasma as 18 and the exhaust stream after the ethanol-containing addition is referred to as 28 designated. The exhaust gas stream treated with both the ozone and the ethanol enters the reduction reactor 24 one.

The reduction catalyst reactor 24 includes a two-pot reduction catalyst. The upstream catalyst bed contains a volume of barium or sodium Y zeolite (indicated as BaY) and the downstream bed, usually of lesser volume, contains copper Y zeolite (indicated as CuY). The temperature at the reactor inlet or in the reactor becomes effective for the operation of the reduction catalyst reactor 24 used to control the plasma energy density and the molar feed ratio of ethanol to NO _x . For example, the temperatures at the inlet and outlet of the reduction catalyst may be monitored for effective exhaust treatment by thermocouples (indicated by T1 and T2) or other suitable temperature sensor (s). The temperature data for controlling the plasma energy density and the amount of ethanol addition are communicated to a digital control device (not shown). The average temperature in the reactor 24 is a function of the temperature of the exhaust gas entering the reactor and the heat of reaction then occurring in the respective beds. The upstream bed is primarily effective at catalyst temperatures of up to about 300 ° C and the downstream bed is more effective at higher catalyst temperatures. The current 30 indicates the treated exhaust gas withdrawn from the exhaust system.

Like in the 2 contains a suitable non-thermal plasma reactor 20 a cylindrical, tubular dielectric body 40 , The reactor 20 has two electrodes, namely a high voltage electrode 42 and a ground electrode 44 which are separated from each other by the tubular dielectric body 40 and an air gap 46 are separated. The high voltage electrode 42 is one along the longitudinal axis of the tube 40 placed rod. The ground electrode 44 is one around the tubular dielectric body 40 wire wound in a spiral pattern. The spiral grounding electrode 44 in combination with the axial high voltage electrode 42 provides more intertwined helical areas more active 54 and more passive 52 electric fields along the length of the reactor 20 , The spiral-shaped, active electric field 54 around the ground electrode 44 around is highly focused for efficient plasma generation and ozone generation.

At the end connections 48 . 50 At the central electrode, a high frequency electrical high voltage potential is applied. The spiral outer grounding electrode 44 is like at 56 indicated grounded. When operating the plasma reactor 20 is causing air in one end of the reactor around the central electrode 42 around and inside the dielectric tube 40 and from the other end in the direction of the in the 2 arrows shown flows. That to the central electrode 42 applied electrical potential generates the previously described active 54 and passive 52 Fields in the reactor 20 , These high frequency high potential fields 52 . 54 generate in the flowing air flow in the air gap 46 reactive oxygen species, resulting in the production of ozone. This ozone-containing air stream leaves the reactor 20 and will, as in the 1 shown inserted directly into the exhaust stream. As will be described in detail below, reference is made to the plasma reactor 20 applied electrical energy in an amount that is an inverse function of the temperature of the exhaust stream. The side stream plasma reactor 20 is close to but away from the hot exhaust pipe.

Ethanol can, as in the 1 indicated, directly from the reservoir 26 be injected into the exhaust stream and is rapidly evaporated to meet its reduction function. Alternatively, air can pass through a reservoir 26 be bubbled with ethanol or with ethanol dissolved in a solvent having a low vapor pressure, such as diesel fuel, to entrain ethanol vapor and carry it into the exhaust gas stream. As will be described further below, the amount of introduced into the exhaust stream ethanol depending on the NO _x content of the exhaust gas and the temperature of the reducing catalyst.

There are four important factors in the practice of the present invention: the non-thermal side stream plasma reactor for the production of ozone from air; the composition of the reduction catalyst; the ethanol reducing agent additive and the management of the working parameters, in particular the Plasma energy and the feed ratio of ethanol / NO _x . The plasma energy density and the feed ratio of ethanol / NO _x vary with the reduction catalyst temperature. The exhaust gas treatment method according to the present invention incorporating these four factors is illustrated using a laboratory structure and an engine dynamometer structure.

Experimental laboratory construction

A laboratory reactor system was designed to reduce the NO _x to N ₂ in a synthetic exhaust gas stream by the method described in US Pat 1 to evaluate illustrated methods. The Laboratory Reduction Catalyst Reactor 24 had a small, upstream-downstream, NaY zeolite / CuY zeolite two-pass reduction catalyst reactor. Provision has been made for the production of a synthetic NO and O 2 -containing off-gas for the plasma-generated ozone treatment, as well as for ethanol and for the flow through the reduction catalyst reactor. This laboratory setup had flexibility for different flow arrangements of gas flow through the plasma reactor and for the location of introduction of ethanol. The experimental conditions for the laboratory reactor system are summarized in Table 1 below. Table 1: Experimental conditions for the laboratory reactor system Catalyst: NaY (9.9 wt% Na) BaY (14.7% by weight Ba) CuY (7.3% by weight of Cu) Supply concentration: NO = 225 ppm HC = 1200-4800 ppm C ₁ O ₂ = 16.8% H ₂ O = 1.6% N ₂ = remainder Temperature: Plasma = 150 ° C Catalyst = 150-400 ° C Plasma: Energy density = 6-20 J / l AC voltage = +/- 7 kV Print: 101.3 kPa Speed: Plasma = 25 K / h, Catalyst = 11 K-44 K / h (NaY, BaY) 22 K-44 K / h (CuY)

For most tests the flow rate of 225 ppm of NO was containing synthetic exhaust gas 143 cm ³ / min. (STP) and the rate of Ethanalzugabe was correspondingly 0.00051 ^cm3 / min. The simulated exhaust gas was obtained by mixing individual gases whose flow velocities were controlled by mass flow controllers to obtain the desired exhaust gas composition. The gas mixture was fed to steam by passing the gas stream through a bubbler containing deionized water at room temperature. The product gas mixture from the reduction catalyst reactor 24 (as in the systems of 1 ) was diluted with an equal volumetric amount of N ₂ to prevent condensation in the gas flow line between the reactor and the analyzer. The chemical compositions of the feed and product streams to and from the reactor were monitored by a FTIR Gas Analyzer (Nicolet Nexus 670). The temperature of the FTIR gas cell was maintained at 165 ° C. The NO _x conversion measured by FTIR became checked by a NO / NO _x chemiluminescence analyzer. Both results were in good agreement with each other within 5%.

The plasma reactor used in the laboratory tests 20 was made of quartz tube with an outer diameter (od) of 3/8 inch with a high voltage electrode 42 made in its center. The ground electrode 44 was a Ni coated copper wire wound around the outer surface of the quartz tube in two complete turns at a distance of 0.2 cm. The total length of the plasma region was 0.4 cm. The high voltage electrode 42 was made of a 1/16 inch od stainless steel rod inserted into an 1/8 inch OD aluminum tubing through the annular air gap 46 Air flowed through to form the plasma. For comparison purposes, instead of ambient air, sometimes a simulated exhaust gas has passed through the plasma reactor 20 passed. A high voltage of +/- 7 kV in a sinusoidal shape and at a frequency of 200 Hz was monitored by a function generator (Tektronix Model AFG 310) and a high voltage amplifier (Trek Model 20 / 20B) monitored by a high voltage probe (Tektronix Model P6015A) was created. The electrical responses of the plasma reactor 20 were monitored by both a digital oscilloscope (Tektronix Model TDS 430A) and a computer, with the digital oscilloscope providing the transient response of the plasma reactor 20 including the time deviations of the applied voltage, the gas discharge current and the energy absorption by the plasma, whereas the computer recorded the average time energy consumption by the plasma. For all laboratory results described here, the inlet temperature of the plasma reactor became 20 kept at 150 ° C. However, the plasma reactor can 20 be operated at ambient temperatures.

The catalyst reactor 24 included a reduction catalyst with two separate catalyst reaction zones. In these laboratory studies, the synthetic exhaust gas was at ambient temperature and the catalyst was heated to a test temperature in the range of 150 ° C to 400 ° C. The first catalyst zone contains barium (or sodium) Y zeolite which occupied about four times the space of the second catalyst zone containing copper Y zeolite. Starting at temperatures below 300 ° C, the following parallel chemical reactions between the ethanol reductant and the NO ₂ over the BaY (or NaY) zeolite catalyst were observed: EtOH + NO ₂ → AA + NO → N ₂ EtOH + NO ₂ → N ₂

As soon as the exhaust gas flowed above the CuY zeolite catalyst at temperatures above 300 ° C, the following reaction took place: EtOH + NO + NO ₂ + O ₂ → N ₂

Both the BaY and CuY catalysts were prepared by aqueous zone exchange of NaY with Ba (NO ₃ ) ₂ and Cu (NO ₃ ) ₂ precursors. All samples in powder form were pelleted and crushed to a size of 30 to 40 mesh. The granulated catalysts were packed in a one quarter inch quartz tube reactor located downstream of the plasma reactor 20 was placed. Then the BaY (or NaY) catalyst was followed by the CuY catalyst. The catalysts were pretreated at 400 ° C for 3 hours in an air stream containing 2.5% steam. The reaction temperature was at the outlet of the reactor 24 monitored and controlled by an electronic temperature control device with an accuracy of approximately +/- 1 ° C.

The chemical composition of the exhaust gas was monitored at three different locations: in front of the plasma reactor, between the plasma reactor and the catalyst reactor and behind the catalyst reactor. A Horiba-made engine exhaust analyzer (Mexa-7500 DEGR) that included a NO / NO _x chemiluminescence analyzer, an HFID HC analyzer, an O ₂ analyzer, and IR analyzers for CO, CO _2, and N ₂ O was used for the chemical analysis. The HFID HC Analyzer was used for all carbon from both HC and organic POHC (partially oxidized hydrocarbons). A conversion of NO _x to N ₂ of about 90% was achieved using the exhaust treatment method of the present invention at a reactor exit temperature of about 300 ° C.

The beneficial effect of using the ethanol-containing reducing agent with the reducing catalyst combination compared to conventional hydrocarbon reducing agents is disclosed in U.S.P. 3 shown. The data was recovered from the exhaust gas streams after reducing NO ₂ to N ₂ using of the BaY zeolite / CuY zeolite reduction catalyst combination. The investigations were carried out using three different reducing agents, namely propylene, gasoline and ethanol. The exhaust gas flow was measured for both the BaY zeolite catalyst and the CuY zeolite catalyst at a space velocity of 55,000 / hr. passed through the reduction catalyst. The ratio of the molar supply of ethanol to NO _x (EtOH / NO _x ) was 18 and the plasma was generated from an air stream with a plasma energy density of 20 J / L. Each reducing agent was injected before the plasma reactor into the main power line (Vorplasmainjektion). Using a syringe pump, denatured ethanol and E-85 (85% ethanol and 15% gasoline) were introduced, whereas gasoline vapor was introduced by flowing air over the surface of liquid gasoline contained in the quartz tube.

Over the operating time of the reduction catalyst ("time to current"), the percent conversion of NO _x for each reducing agent tested was recorded in minutes. Like in the 3 with ethanol as the reducing agent, higher conversion of NO _x over the operating time of the reduction catalyst was achieved, whereas the other reducing agents tested (propylene and gasoline) resulted in initially higher NO _x conversions, but these rapidly degraded the catalyst by coking. In the case of propylene, the NO _x conversion initially increased for about 20 minutes before it began to gradually decrease from white to slightly brown with the concomitant change in the catalyst color at the front of the catalyst bed. With gasoline, the catalysts began to lose their activity immediately by changing the face of the catalyst bed to a gray-black color. These findings indicate that catalyst deactivation is due to coke formation on the surface of the catalyst during the SCR process. Interestingly, the catalyst activity with ethanol improved with the reaction time ("time-on-stream") and the face of the catalyst bed remained as white as the original color. This absence of catalyst deactivation with ethanol encourages the use of HC containing ethanol and ethanol (such as E-diesel containing 85% diesel fuel and 15% ethanol or E-85 containing 85% ethanol and 15% gasoline) as reducing agents for NO _x reduction in exhaust treatment systems ,

Laboratory tests were also conducted to determine a preferred order for the introduction of the ozone-containing plasma and ethanol into the NO _x -containing exhaust gas stream. In these tests, E-85 was injected using a syringe pump. In some tests, the E-85 was injected into the exhaust stream before the ozone-containing plasma was introduced into the exhaust stream, presumably allowing some of the ozone to react with ethanol. In other tests, after introduction of the ozone-containing plasma, the ethanol was injected into the exhaust stream so that only the simulated exhaust mixture was treated by ozone-treated plasma.

It was found that injection of E-85 in front of the plasma reactor produced more acetaldehyde (AA) but less NO ₂ than when E-85 was added after the plasma reactor. The amount of AA produced was approximately 10% of the ethanol feed, indicating that the efficiency of the plasma to convert ethanol to AA is low. It was also concluded that the presence of ethanol in the exhaust gas stream suppresses the conversion of NO to NO ₂ by the plasma. Accordingly, the laboratory studies indicate that the ethanol is preferably introduced into the NO _x exhaust stream downstream of the introduction of the ozone-containing plasma.

Laboratory tests have also been conducted to (a) provide the amount of ethanol needed for the catalytic reduction of NO _x content over a catalyst temperature range above 150 ° C to 400 ° C and (b) the energy requirements for the non-thermal plasma reactor over the same temperature range determine. The data obtained in these tests are in the 4 summarized. The data in the 4 show opposite trends for the feed ratio of EtOH / NO _x and the plasma energy density for optimum system performance over a temperature range of the BaY-CuY twin catalyst between 150 ° C and 400 ° C.

The solid line in the 4 shows that at lower catalyst temperatures (ie at engine warm-up temperatures) more plasma energy is needed. The data shows that an amount of energy of about 18 J / l is needed over a catalyst temperature range of 150 ° C to 200 ° C and thereafter the amount of energy required is substantially linearly reduced so that in these synthetic exhaust tests at about 400 ° C no plasma energy is needed. Thus, it appears that a plasma-free oxidation environment in the exhaust gas at these higher temperatures is suitable for oxidizing NO to NO ₂ for reduction over the base metal divalent zeolite catalyst.

These laboratory tests also indicate that with increasing catalyst temperature, a higher molar ratio of ethanol to NO _x content is needed. Like in the 4 As shown, the optimum molar feed ratio of ethanol to NO _x content gradually increases from about 5 to about 25 over a range of catalyst temperatures between 150 ° C and 400 ° C. This is obviously a result of the fact that in the highly oxidizing environment more ethanol is oxidized by oxygen before it can serve as a reducing agent for NO _x .

In the laboratory tests, an average NO _x conversion of 91% over a temperature range of 200 ° C to 400 ° C was obtained under optimum operating conditions when a twin catalyst reactor containing NaY and CuY was used together with ethanol (E85) as the reducing agent. The simulated exhaust gas mixture contained 215 ppm NO and its space velocity (SV) through the NaY bed was 11 K / hr. and their SV through the CuY bed was 22 K / hr. The NO _x conversion data (%) over a temperature range of 150 ° C to 400 ° C are graphically in the 5 shown. The optimum operating conditions used in the laboratory tests were as follows: the feed ratio of hydrocarbon to nitrogen oxides (HC / NO _x ) was varied with the catalyst temperature (eg 24 at 400 ° C, 12 at 300 ° C and 6 at 200 ° C) while the plasma energy was kept constant at 12 J / l over the catalyst temperature range between 150 ° C and 350 ° C and turned off at 400 ° C.

Experimental dynamometer setup

The tests were carried out on an engine dynamo with a 6-cylinder diesel engine with a displacement of 4.9 l. The engine was operated under controlled conditions to produce an exhaust gas to be treated by the present process. The experimental setup is in the 6 shown. This allowed flexibility in terms of flow volume, direction of flow and chemical analysis of the various flow streams. The volume of the diesel exhaust gas was greater than the capacity of the reduction catalyst reactor prepared for these experiments. Therefore, only part of the exhaust gas was used for these tests. Accordingly, a side stream pipe (S) was taken from the main exhaust pipe (M), and its flow velocity was as in FIG 8th shown using a check valve set. The side stream (S) was then used in the following experiments. The engine specification and experimental conditions for the dynamometer tests are summarized in Table 2. Table 2. Typical experimental conditions for engine dynamometer tests Engine: 4.9 1/6 cylinder Isuzu engine Speed 2000 rpm Torque = 60 Nm EGR = 0 Fuel = Chevron ULS (0.2 ppm S) Swedish (2 ppm S) Amoco Premium (400 ppm S) Exhaust gas temperature = 240 ° C Exhaust gas composition: HC = 180 ppm C ₁ NO _x = 235 ppm O ₂ = 16.7% CO ₂ = 3.3% CO = 0.11% Plasma reactor at the side stream: Cylindrical geometry SV = 60 K / h. Power density (E _p ) = 18 J / l Temperature (T _p ) = 25 ° C Reduction catalyst reactor on the side stream: Catalysts = monoliths (NaY, BaY, CuY, DOC) Reactor = stainless steel tube with 2 '' od Reducing agent: E-diesel, ethanol SV = 5 K-10 K / h. for NaY, BaY 20K-40K / hr. for CuY Temperature (T _c ) = 150-400 ° C Exhaust gas flow from the engine: Flow rate = 10-20 l / min.

The withdrawn diesel exhaust stream was first passed over a palladium-based diesel oxidation catalyst (DOC) and a ceramic filter before contacting it with the twin catalysts (BaY + CuY). Analysis of the oxidized stream by analyzer (AN) indicated that part of the acetaldehyde was formed during its oxidation. The acetaldehyde was considered to be a suitable reducing agent for reducing NO _x to N ₂ . Both the hyperplasma reactor (HP) and the e-diesel reservoir (E) were located outside the engine exhaust stream S.

To set higher exhaust stream flow rates used in the engine dynamometer tests, a larger plasma reactor HP was made by extending the plasma forming area of the laboratory reactor while maintaining the geometry and diameter thereof. That is, the ground electrode was made of a Ni-coated copper wire wound in twenty complete turns around the outer surface of the quartz tube at a pitch of 0.2 cm, resulting in a total plasma-forming area length of 4 cm. The electrical frequency was also increased to 2000 Hz at +/- 7 kV. The ozone-containing stream (SS) was injected into the exhaust gas stream S.

In the treatment of the dynamometer exhausts, either pure ethanol or ethanol vaporized by air from an E-diesel mixture was injected into the exhaust stream. The e-diesel (a solution of 85% diesel fuel and 15% ethanol) was held in the reservoir E and air was bubbled through the solution to strip substantially pure ethanol vapor from the high vapor pressure diesel fuel and via the stream B in to carry the exhaust stream S. The diesel fuel in the ethanol containing E diesel mixture served as a carrier for transferring the reductant to the exhaust gas stream. To maintain a high level of NO _x conversion, the required amount of ethanol could account for about 0.1 to 1.0% of the total exhaust stream composition.

It was found that the ethanol-containing air stream leaving the reservoir at room temperature was saturated with ethanol vapor. In the experimental setup, the air stream containing ethanol (stream B) in the side stream S could be injected into the exhaust stream after the ozone-containing plasma based on 10 l / min. of the exhaust flow in side stream S, at a rate of 0.037 cm ³ / min. into the exhaust was injected with approximately 235 ppm NO _x . It has also been provided for comparison purposes to introduce ethanol-saturated air into the plasma reactor HP (stream A) together with the air stream to be treated in the HP.

The reduction catalyst was prepared from honeycomb shaped NaY zeolite (JohnsonMatthey) coated cordierite stones (5.66 inches in diameter by 6 inches in length). Cylindrical core pieces with a diameter of 1.875 inches were drilled out using a 2 inch hole drill. BaY and CuY versions of the honeycomb-shaped stones were prepared by aqueous ion exchange of the NaY coated stones with Ba (NO ₃ ) ₂ and Cu (NO ₃ ) ₂ precursors. The stones were then calcined for four hours at 500 ° C under atmospheric conditions. The honeycomb catalysts were wrapped with an insulating cloth before being introduced into a 2 inch OD stainless steel reactor tube. In the two-bed reactors, the NaY coated (or BaY coated) stones were followed downstream of CuY coated stone. The reactor temperatures were at both the inlet and monitored at the outlet of the reactor with a typical accuracy of +/- 1 ° C. These temperatures varied with the temperature of the exhaust stream from the diesel engine.

The chemical composition of the exhaust gas was monitored by an analyzer AN at three different locations: before the introduction of the ozone plasma from the plasma reactor, after the plasma introduction (but before the catalyst reactor) and after the catalyst reactor. For chemical analysis, a Horiba-made engine exhaust analyzer (Mexa-7500 DEGR) incorporating a NO / NO _x chemiluminescence analyzer, an HFID HC analyzer, an O ₂ analyzer and IR analyzers for CO, CO ₂ and N ₂ O was used. The HFID HC analyzer was used for the total carbon of both HC and organic POHC. Using the exhaust treatment process of the present invention at a reactor exit temperature of about 300 ° C, an approximately 98% conversion of NO _x to N _{2 was} achieved.

The effect of the ozone-containing plasma (essential to the practice of the present invention) on the performance of the NO _x reduction process is disclosed in _U.S.P. 7 shown. The data was taken from the stream S after the exhaust gas at a space velocity of 5000 / hr. through a monolith of BaY zeolite and then at a space velocity of 20,000 / hr. passed through a monolith of CuY zeolite. The engine exhaust was produced using Chevron ULS fuel in which HC and CO were removed using a palladium oxidation catalyst DOC.

Like in the 7 For example, the conversion of NO _x to N ₂ versus the temperature of the twin catalyst was plotted in degrees Centigrade. The solid line indicates NO _x conversion using an ozone-containing plasma ("plasma on"), the plasma being generated using a plasma energy of 18 J / l. The broken line represents NO _x conversion without the use of plasma ("plasma off"). At a typical catalyst temperature (about 300 ° C), with "plasma on" a higher conversion of NO _x (about 98%) can be achieved, whereas a much lower conversion (about 78%) with "plasma off" is achieved. For the "plasma on" case, NO _x conversion decreases with increasing temperature above a threshold of 300 ° C, the threshold at which maximum conversion is achieved. Although the conversion of NO _x with the "plasma off" reduction method increases with increasing temperature, the "plasma off" case at each temperature tested does not achieve as high a conversion of NO _x as the "plasma on" case. As can be seen from the diagram, the NO _x conversion improves when the plasma energy is off, with increasing catalyst temperature.

The effect of the gas space velocity in the reduction reactor on the NO _x conversion efficiency of the NO _x reduction system according to the present invention was also tested using NaY zeolite as the reduction catalyst. The results showed that the NO _x conversion efficiency increased with decreasing space velocity over a temperature range of 200 ° C to 400 ° C and at 5000 / hr. and 200 ° C reached a 90% conversion. The average rate of change of NO _x conversion with the catalyst temperature for a space velocity of 10,000 / hr. is about twice as high as for 5000 / hr, which is consistent with kinetic theory. The laboratory reactor data was then taken with the engine dynamometer test data in which the laboratory setup used a NaY powder catalyst at a space velocity of 11000 / hr. contained, compared. The performance of the dynamometer test using real engine exhaust at a space velocity of 5000 / hr. The honeycomb catalyst employed is approximately the same as that of the laboratory simulated exhaust stream at a space velocity of 11,000 / hr. used powder catalyst. This shows that a catalyst volume ratio of 2.2 is applicable between the powder catalyst and the honeycomb catalyst for the same performance. Consequently, the bulk volume of the honeycomb catalyst must be about twice that of a powder catalyst to maintain the same performance.

The 8th compares the efficiency of two different E-diesel injection modes for _NOx conversion performance: pre-plasma and Nachplasmainjektion. In the preplasma injection, the ethanol (stream A in the 6 ) was injected into the airflow entering the HP and thus was treated in the hyperplasma reactor as part of the SS flow entering the exhaust. In the post-plasma injection, the ethanol (stream B in the 6 ) is injected downstream of the plasma injection into the exhaust, stream SS. Like in the 8th shown, reaches the Nachplasmainjektion of the E-diesel at both 200 ° C and at 300 ° C a slightly better NO _x conversion than the Vorplasmainjektion.

Another way of examining the efficiency of the catalysts, the temperature change of the catalyst bed, was compared as follows. First, the (plasma + catalyst) system was stabilized with post-plasma injection of E-diesel until it reached a steady state such that the temperature at the catalyst exit remained at the desired temperature (ie, 200 or 300 ° C). Then, the E-diesel injection mode was switched to the pre-plasma injection mode during which the temperature changes due to this switching from the post-plasma injection to the pre-plasma injection were monitored. For both 200 ° C and 300 ° C, there is a substantial increase in temperature due to this switching with concomitant increases in CO and CO ₂ levels. This is an indication of increased oxidation (combustion) of the E-diesel in the Vorplasmainjektion compared to the Nachplasmainjektion resulting in a reduced NO _x conversion performance, although the difference is small. Based on these observations in both the laboratory test and the dynamometer tests, it is concluded that the ethanol-based reducing agents need not be plasma-treated for efficient NO _x reduction. Preferably, the ethanol is separated in the required amount and introduced downstream of the ozone-containing plasma injection.

The method according to the present invention is applicable to the use of any type of fuel used in diesel engines regardless of the sulfur content present in the fuel. Experiments were conducted using three different fuels including Chevron ULS, Swedish and Amoco Premium with 0.2 ppm sulfur (S), 2 ppm S, and 400 ppm S, respectively. At a typical catalyst temperature of 300 ° C, NO _x conversion is not significantly affected by the sulfur content present in the fuel.

Claims (14)

A method for reducing nitrogen oxides, including NO, in an exhaust gas stream also containing oxygen, carbon monoxide, and hydrocarbons at a temperature above 150 ° C, the method comprising: flowing air through a non-thermal plasma reactor to produce an ozone-containing plasma, and feeding the plasma to the exhaust gas stream for the oxidation of NO to NO ₂ , supplying ethanol to the exhaust gas stream separate from the supply of the plasma for the reduction of the nitrogen oxides and then contacting the exhaust gas stream with a Zweibettreduktionskatalysator contained in the first bed NaY zeolite and / or BaY zeolite and in the second bed CuY zeolite to reduce the nitrogen oxides to N ₂ . A method of reducing nitrogen oxides according to claim 1, wherein ethanol is supplied to the exhaust gas stream downstream of the supply of the ozone-containing plasma. The method for reducing nitrogen oxides according to claim 1, wherein the plasma reactor is a tubular vessel having a reactor space for passing air, the plasma reactor includes a high voltage electrode disposed in the reactor space and a ground electrode spirally wound around the tubular vessel to thereby to provide interlaced helical passive and active electric fields for the generation of the ozone-containing plasma. A method of reducing nitrogen oxides according to claim 1, wherein the flow of the exhaust gas through the first bed is at a space velocity which is higher than the space velocity of the exhaust gas flow through the second bed. A method of reducing nitrogen oxides according to claim 1, comprising supplying ethanol to the exhaust stream as ethanol vapor in an air stream. A method of reducing nitrogen oxides according to claim 5, comprising passing an air stream through a solution of ethanol to produce ethanol vapor in the air stream. A method of reducing nitrogen oxides according to claim 6, wherein the ethanol is dissolved in gasoline and the ethanol constitutes at least 85% by volume of the solution. A method of reducing nitrogen oxides according to claim 6, wherein the ethanol is dissolved in diesel fuel and the ethanol constitutes 1 to 15% by volume of the solution. A method of reducing nitrogen oxides according to claim 1, comprising operating said plasma reactor at a plasma energy density in a range between 0 and 20 joules / liter (J / l) as a function the catalyst temperatures in a range between 150 ° C and 400 ° C, wherein the plasma energy density at catalyst temperatures of 350 ° C or higher is zero or reduced to zero. A method of reducing nitrogen oxides according to claim 1, wherein ethanol is added to the exhaust stream in a molar ratio of ethanol to nitrogen oxides (EtOH / NO _x ) in a range between 5 and 25 as a function of catalyst temperatures in a range between 150 ° C and 400 ° C is supplied. A method of reducing nitrogen oxides, NO _x including NO, in a diesel engine exhaust stream also containing oxygen, carbon monoxide, and hydrocarbons at a temperature above 150 ° C, the method comprising: (a) passing the exhaust stream into contact with an oxidation catalyst for carbon monoxide and oxidizing the hydrocarbons, (b) passing air through a non-thermal plasma reactor to produce an ozone-containing plasma, and supplying the plasma into the exhaust gas stream for the oxidation of NO to NO ₂ , wherein the energy applied in the plasma reactor Temperatures in a range between 150 ° C and 400 ° C inversely proportional to the temperature of the reduction catalyst, wherein the energy is reduced to zero at temperatures of 350 ° C or higher, (c) supplying ethanol to the exhaust stream for the reduction of nitrogen oxides , wherein the amount of ethanol in relation to the reduction of Katal and (d) passing the exhaust gas stream through a two-pass reduction catalyst reactor, the reactor containing a first bed consisting of barium and / or sodium Y zeolite catalyst and a second bed consisting of copper Y zeolite catalyst, wherein the volume of the first bed is greater than the volume of the second bed. The method of claim 11, wherein the plasma reactor is a tubular vessel having a reactor space therein for a flow passage therein, the plasma reactor containing a high voltage electrode disposed in the reactor space and a ground electrode spirally wound around the tubular vessel to thereby spiral into each other to provide passive and active electric fields for the generation of the ozone-containing plasma. The method of claim 11, comprising vaporizing ethanol and supplying the ethanol as vapor in an air stream to the exhaust gas stream. The method of claim 11, comprising evaporating ethanol from e-diesel at the exhaust gas stream ethanol in a molar ratio of ethanol to nitrogen oxides (EtOH / NO _x ) in a range between 5 and 25 as a function of the catalyst temperatures in a range between 150 ° C. and 400 ° C is supplied.

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